The American Academy of Pediatrics (AAP) and the National Highway Traffic Safety Administration (NHTSA) recommend to use the harness for as long as possible, until it has genuinely been outgrown. NHTSA even recommends that 4-7 year old children use the harness for as long as they can: http://www.safercar.gov/parents/RightSeat.htm
A booster seat can only protect a child who sits straight up and back into the seat. It would be absolutely terrible if a child were to lean over to pick up their toy or to unbuckle themselves at the time of a crash. Generally, most children are ready to use a booster seat when they are around 5-6 years old.
A child under the age of 4 should never be in a booster seat. This is because the older a child becomes, the more developed their bones are. A child under the age of 4 does not have the bone density to take the force of a lap/shoulder seat belt. A 5 point harness distributes the crash forces more evenly over a child's body.
Some states like Georgia require children to be both and at least 4 years old and 40 lbs before they can transition into a booster seat that uses the lap/shoulder seat belt.
A forward facing car seat with a harness is outgrown when ONE of the following happens:
-the child has hit the maximum weight limit with the harness
-the tips of the child's ears are even with the shell
-the harness is parallel to the child's shoulders*
*When front facing, it is very important that the harness straps are at or above the child's shoulders. Most children outgrow their harnessed car seats by height before weight. This is why it is recommended to get car seats with tall top harness slots, greater than 17 inches to ensure that a child can use the harness function of a car seat for as long as possible.
If your child has outgrown their harnessed car seat and they are not ready for a booster seat, I'd recommend the Evenflo Maestro (50 lb weight limit with harness), Evenflo SecureKids 300 or 400 model (65 lbs weight limit with harness), and the Graco Nautilus (65 lb weight limit with harness). All of these seats have about 18" top harness slots.
The Britax Frontier 85 goes up to 85 lbs with the harness as long as you use the top tether and has about 20" top harness slots. If your vehicle does not have a top tether anchor, it is recommended to get one installed. The top tether is extremely important on any forward facing car seat as it reduces head excursion significantly.
A booster seat is designed to boost a child up for a better seat belt fit so that the lap belt is on the child's hips and the shoulder belt between the child's neck and collar bone. A seat belt that is improperly positioned on a child's body will severely injure or kill a child. Some boosters make seat belts fit worse. The Insurance Institute for Highway Safety (IIHS) rates how well booster seats position the adult seat belt. Here are their findings. If you don't know the specific model, you can sort the list out by brand name and their are pictures to help guide you: http://www.iihs.org/research/topics/boosters/default.html
Here is more about seat belt fit: http://www.boosttil8.org/
If your child is at least 5-6 years old, and your combination seat does not properly position the seat belt, then I'd recommend the Graco Turbobooster ($50) or the Evenflo Big Kid (in highback) ($30) Both of these models recieved "best bet" ratings from the IIHS, which mean they do a great job positioning the seat belt on the average 6 year old child.